Hydrodynamics of Sediment Retention Ponds

Abstract

Sediment retention ponds are used on construction sites to manage sediments and to protect the downstream ecosystem. Treatment aims to improve water quality by reducing the amount of suspended solids by settling, and the hydraulics of sediment retention ponds are crucial for this process. A review of the literature indicated that understanding the hydraulics of sediment retention ponds is critical to their optimal performance. Their hydraulic design has received limited attention, however, and consequently the current design of New Zealand sediment retention ponds is more focused on the hydrology of the catchment. The study presented in this thesis investigated the hydraulics of sediment retention ponds using dye tracer studies and Particle Tracking Velocimetry (PTV) techniques to directly quantify the internal flow pattern. The investigation included the construction of a physical scale model of a field retention pond in the laboratory and simulations using computational fluid dynamics (CFD) modeling. The model pond was designed as a 1:10 scale replica of an existing retention pond situated at the Alpurt B2 Motorway site, north of Auckland, New Zealand. This study is the first to investigate how the arrangement of floating treatment wetlands (FTWs) in a retention pond affects its hydraulic performance. Experimental investigations were undertaken to optimize the layout of FTWs for maximum hydraulic performance. A CFD model was assessed against experimental flow data from the laboratory model to test the suitability of the numerical model for the hydraulic design of sediment retention ponds. Scaling issues in hydraulic modeling of sediment retention ponds were also investigated using CFD modeling, with the aim of devising a methodology for selection of an appropriate size of physical model for accurate representation of the hydrodynamics of field sediment retention ponds. Experimental findings and subsequent analysis revealed that installing an FTW in a retention pond significantly improves the pond's hydraulic performance. The hydraulic performance was found to depend on the position, size and placing arrangement of the FTW in the pond, as well as on the inlet arrangement. Similarly, it was found that the inclusion of baffles and deflector islands may exacerbate short-circuiting in the pond. This unexpected behaviour, in relation to previous studies, is considered to be a consequence of the model pond incorporating sloping walls, which is a novel aspect of this study. However, the results reported here are limited to small and narrow ponds where a large portion of the pond is batter. On large ponds, batter effects may be neglected or are different from those reported here. The results of this study identify a number of methods that represent significant advances in the field of hydraulic design of sediment retention ponds. Most importantly, this study provides design guidelines for engineers to retrofit FTWs in existing ponds, as well as newly constructed ponds. This research therefore addresses a major knowledge gap in the design of FTWs retrofitted in sediment retention ponds in order to optimize their hydraulic efficiency. Although, the water quality treatment aspect of FTWs has been well documented, currently there are no definitive design guidelines available to achieve optimal hydraulic performance of FTWs. In this context, this study is the first of its kind. Another important contribution of this research is its investigation of the potential of CFD modeling in this context; its use to simulate any unforeseen problems with the design of sediment retention ponds is validated.